Longitudinal analysis and visualization under limited accuracy system

ABSTRACT

Processing logic makes a comparison between first image data and second image data of a dental arch and determines a plurality of spatial differences between a first representation of the dental arch in the first image data and a second representation of the dental arch in the second image data. The processing logic determines that a first spatial difference is attributable to scanner inaccuracy and that a second spatial difference is attributable to a clinical change to the dental arch. The processing logic generates a third representation of the dental arch that is a modified version of the second representation, wherein the first spatial difference is removed in the third representation, and wherein the third representation comprises a visual enhancement that accentuates the second spatial difference.

RELATED APPLICATIONS

This patent application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application No. 62/460,707, filed Feb. 17, 2017, andfurther claims the benefit under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication No. 62/492,047, filed Apr. 28, 2017, both of which areincorporated by reference herein.

TECHNICAL FIELD

Embodiments of the present invention relate to the field of dentistryand, in particular, to a system and method for performing longitudinalanalysis and visualization of an oral cavity (e.g., a dental arch in anoral cavity) under a limited accuracy system.

BACKGROUND

Dental practitioners generally make assessments of clinical problems ina patient's oral cavity based on visual inspection and personalknowledge. However, small changes to tooth and/or gum surfaces can haveclinical importance, and it can be difficult for the dental practitionerto identify such small changes. Additionally, the magnitude and rate ofchange to a patient's dentition may not be easily determined. Forexample, the dental practitioner may have difficulty in determining thespecific, subtle changes that might have occurred to the patient'sdentition.

Intraoral scanners are a useful tool in dentistry and orthodontics.However, intraoral scans are not generally used for longitudinalanalysis. One reason is that the results of intraoral scans performed byintraoral scanners include errors introduced by the intraoral scanners,which makes comparison between images difficult and error prone.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not by way of limitation, in the figures of the accompanyingdrawings.

FIG. 1A illustrates one embodiment of a system for identifying dentalissues, in accordance with an embodiment.

FIG. 1B illustrates one embodiment of a dental issue identifier, inaccordance with an embodiment.

FIG. 2 illustrates a flow diagram for a method of identifying dentalissues, in accordance with an embodiment.

FIG. 3A illustrates a flow diagram for a method of registering imagedata of a dental arch and identifying and classifying dental issues, inaccordance with an embodiment.

FIG. 3B illustrates a flow diagram for a method of registering imagedata of a dental arch and identifying and visualizing dental issues, inaccordance with an embodiment.

FIG. 4A is a diagram illustrating first image data for a dental arch, inaccordance with an embodiment.

FIG. 4B is a diagram illustrating the dental arch of FIG. 4A afterpoints on a surface of the dental arch are selected, in accordance withan embodiment.

FIG. 4C is a diagram illustrating the dental arch of FIG. 4B showing aparticular region around a selected point on the surface, in accordancewith an embodiment.

FIG. 4D is a diagram illustrating matching of the particular region ofFIG. 4C to second image data for the dental arch, in accordance with anembodiment.

FIG. 4E illustrates a mapping of points on a test surface tocorresponding points on a reference surface.

FIG. 4F is a diagram illustrating a warp space transformation betweenfirst image data of a dental arch and second image data of the dentalarch, in accordance with an embodiment.

FIGS. 5A-5D illustrate visual maps showing correlation between firstimage data of a dental arch and second image data of the dental arch, inaccordance with an embodiment.

FIG. 6 illustrates a flow diagram for a method of determining appearancedifferences between first image data of a dental arch and second imagedata of the dental arch, in accordance with an embodiment.

FIG. 7A illustrates a tooth before and after a change in appearance ofthe tooth.

FIG. 7B illustrates a set of teeth before and after a change inappearance of one of the teeth.

FIG. 8 illustrates a flow diagram for a method of generating arepresentation of a dental arch that accentuates changes that haveoccurred in the dental arch, in accordance with an embodiment.

FIG. 9 illustrates extrapolated positions of a set of points on a dentalarch.

FIG. 10 illustrates a flow diagram for a method of generating a visualoverlay for a virtual model of a dental arch, in accordance with anembodiment.

FIG. 11 is a diagram illustrating labeling of differences between actualtooth movement and planned tooth movement for a dental arch, inaccordance with an embodiment.

FIG. 12 illustrates a table showing which teeth have positions that arein compliance with an orthodontic treatment plan and which teeth havepositions that deviate from the orthodontic treatment plan, inaccordance with an embodiment.

FIG. 13 illustrates a dental arch with a visual overlay indicating whichteeth have moved less than expected and which teeth have moved asexpected, in accordance with an embodiment.

FIG. 14A illustrates a comparison between two different scans of adental arch, in accordance with an embodiment.

FIG. 14B illustrates a representation of a dental arch showing changesthat have occurred to the dental arch over time, in accordance with anembodiment.

FIG. 15 illustrates a block diagram of an example computing device, inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION

Described herein are methods and apparatuses for identifying changesthat have occurred in a dental arch over time that have clinicalsignificance. Many changes of clinical importance may occur to a dentalarch over time, such as tooth movement, gum recession, gum swelling,tooth wear, tooth discoloration, changes in tooth translucency, gumdiscoloration, changes in occlusion, and so on. A single image or singleintraoral scan may not be sufficient to identify dental issues.Additionally, a single image or single intraoral scan will not provideinformation such as whether gum recession or tooth wear has stopped oris continuing, a rate of such gum recession or tooth wear, and so on.However, it can be difficult to detect clinical dental issues based on acomparison between different images or intraoral scans.

Image data such as the image data generated from an intraoral scan of apatient's dental arch often includes errors. For example, intraoralscanners have a limited field of view (FOV), and intraoral images fromintraoral scanners are stitched together to form a three dimensional(3D) image or virtual model of a dental arch (or portion of a dentalarch) that is much larger than the FOV. Such stitching together of theintraoral images causes errors to accumulate. The term virtual model asused herein refers to a model that is in a digital format (e.g., asopposed to a physical or real-world model). Virtual models may be 2Dvirtual models or 3D virtual models. If a virtual model is not specifiedas a 2D or 3D virtual model, then it may be either a 2D virtual model ora 3D virtual model. A virtual 3D model in embodiments may include a 3Dsurface as well as appearance properties mapped to each point of the 3Dsurface (e.g., the color of the surface on each point). Individual 3Dscans and/or individual images may be taken during a scan and used tocreate the virtual 3D model.

The further apart two points are, the greater the accumulated errorbetween them. Additionally, the curved shape of the dental arch and jawcauses specific error modes such as expansion of the distance betweenmolar endings. When two 3D images or virtual models are produced, theaccumulated errors may be different for each of these 3D images orvirtual models. This can make comparison of these two images or virtualmodels difficult, and the differences from errors can drown out or hideclinically significant changes to the dental arch and render suchclinically significant changes undetectable. Additionally, someclinically significant changes may obscure other smaller clinicallysignificant changes. Accordingly, even in the absence of scannerinaccuracy or other error, real intraoral changes like tooth movement ofa tooth can drown out or hide tooth wear for that tooth. For example,large-scale changes such as tooth movement or jaw expansion may hidesmaller changes such as tooth wear or gum recession. Embodimentsdiscussed herein identify and separate out the differences betweenimages or virtual models of a dental arch that are attributable toscanner inaccuracy, and accurately identify additional differencesbetween the images or virtual models that are clinically significant.Additionally, embodiments identify and separate out different classes ofclinically significant changes to prevent any of those changes frombeing hidden by other clinically significant changes. Accordingly, smallchanges (such as those associated with tooth wear, gum recession and gumswelling) that may be clinically significant in dentistry are detectablein embodiments in spite of differences caused by scanner inaccuracy.Additionally, clinically significant changes are also detectable inembodiments in spite of larger clinically significant changes. Thus,false alarms may be reduced and missed detections of clinicallysignificant changes may be avoided by limiting the changes to possibleclinical changes, removing scanner inaccuracies, and/or separatelyidentifying large and small scale clinically significant changes.

It is just as difficult to compare appearance changes (e.g., colorchanges, transparency changes, reflectivity changes, spots, etc.) on atooth if that tooth is moving as it is to compare small scale changes(e.g., due to tooth wear, gum recession, etc.) for that tooth if thattooth is moving. Part of the difficulty in identifying appearancechanges is scanner imperfections related to estimating appearance.Additionally, appearance detected by the scanner may be affected byexternal issues such as view angle, distance, amount of saliva, and soon. Such imperfections should also be compensated for. Embodimentsdiscussed herein further enable such appearance changes to be detectedeven where there is scanner inaccuracy and where larger scale clinicallysignificant changes have occurred (such as tooth movement) that mightotherwise hide or obscure such appearance changes. In embodiments, atooth motion is detected and compensated for, and after the compensationprocessing logic is able compare tooth appearance (e.g., tooth color) todetermine changes in the tooth appearance and/or compute tooth wear.Such comparison may be performed using a generated model of “stationary”teeth where motion was compensated and cancelled out.

In one example embodiment, a method of identifying clinical dentalissues includes making a comparison between first image data of a dentalarch and second image data of the dental arch. The first image data maybe generated based on a first intraoral scan of the dental archperformed at a first time by a first intraoral scanner and the secondimage data may be generated based on a second intraoral scan of thedental arch performed at a second time by the first intraoral scanner ora second intraoral scanner. The method further includes determining aplurality of spatial differences between a first representation of thedental arch in the first image data and a second representation of thedental arch in the second image data. The method further includesdetermining that a first spatial difference of the plurality of spatialdifferences is attributable to scanner inaccuracy of at least one of thefirst intraoral scanner or the second intraoral scanner and that asecond spatial difference of the plurality of spatial differences isattributable to a clinical change to the dental arch. The method furtherincludes generating a third representation of the dental arch that is amodified version of the second representation, wherein the first spatialdifference is removed in the third representation, and wherein the thirdrepresentation includes a visual enhancement that accentuates the secondspatial difference. The visual enhancement may include a visual overlay(e.g., a color overlay) that identifies regions of the dental archassociated with the second difference. The visual enhancement mayadditionally or alternatively include an extrapolation of the spatialdifference into the future to show a more extreme future difference.

In a further example embodiment, the method of identifying clinicaldental issues may include making an additional comparison between thefirst image data and either the second image data and/or the thirdrepresentation. The additional comparison may be performed to identifyappearance differences (e.g., visual differences in color, hue,intensity, and so on) between the first image data and the second imagedata and/or third representation. These appearance differences may bedivided into appearance differences attributable to scanner inaccuracyand appearance differences attributable to clinical changes. Theappearance differences attributable to scanner inaccuracy may beremoved, and the third representation may be updated to accentuate theappearance differences attributable to clinical changes of the dentalarch.

Embodiments are discussed herein with reference to comparison of tworepresentations of a dental arch (e.g., based on intraoral scans takenat two different times). However, it should be understood that more thantwo representations of a dental arch may be compared in embodiments. Forexample, in some embodiments three representations of a dental arch,four representations of a dental arch, or even more representations of adental arch may be compared. Such additional comparisons may be used todetermine if a clinical problem is accelerating or decelerating.Additionally, such additional comparisons may be used to determine if aclinical problem that was previously sub-treatable (not severe enough towarrant treatment) has passed a threshold and should be treated.Additionally, additional comparisons using three or more representationsof a dental arch may be used to improve detectability of slow changingclinical issues and distinguish such slow changing clinical issues fromfalse alarms. In example, if n intraoral scans are performed (where n isan integer), then the results of each of the n scans may be comparedeither to the results of the preceding and subsequent scans or to theresults of every other scan. For example, results of the second scan maybe compared to results of the third scan and the first scan, results ofthe third scan may be compared to results the second scan and the fourthscan, and so on. This may reduce false alarms and improve detection.

FIG. 1A illustrates one embodiment of a system 100 for identifyingclinical dental issues. In one embodiment, system 100 carries out one ormore operations of below described methods 200, 300, 600, 800 and/or1000. System 100 includes a computing device 105 that may be coupled toa scanner 150, an additional image capture device 160, a network 170and/or a data store 110.

Computing device 105 may include a processing device, memory, secondarystorage, one or more input devices (e.g., such as a keyboard, mouse,tablet, speakers, or the like), one or more output devices (e.g., adisplay, a printer, etc.), and/or other hardware components. Computingdevice 105 may be connected to data store 110 either directly (as shown)or via network 170. The network 170 may be a local area network (LAN), apublic wide area network (WAN) (e.g., the Internet), a private WAN(e.g., an intranet), or a combination thereof. The computing device 105may be integrated into the scanner 150 or image capture device 160 insome embodiments to improve mobility.

Data store 110 may be an internal data store, or an external data storethat is connected to computing device 105 directly or via network 170.Examples of network data stores include a storage area network (SAN), anetwork attached storage (NAS), and a storage service provided by acloud computing service provider. Data store 110 may include a filesystem, a database, or other data storage arrangement.

In some embodiments, a scanner 150 (e.g., an intraoral scanner) forobtaining three-dimensional (3D) data of a dental site in a patient'soral cavity is operatively connected to the computing device 105.Scanner 150 may include a probe (e.g., a hand held probe) for opticallycapturing three dimensional structures (e.g., by confocal focusing of anarray of light beams). One example of such a scanner 150 is the iTero®intraoral digital scanner manufactured by Align Technology, Inc. Otherexamples of intraoral scanners include the 3M True Definition Scannerand the Cerec Omnicam manufactured by Sirona®.

The scanner 150 may be used to perform an intraoral scan of a patient'soral cavity. An intraoral scan application 108 running on computingdevice 105 may communicate with the scanner 150 to effectuate theintraoral scan. A result of the intraoral scan may be a sequence ofintraoral images that have been discretely generated (e.g., by pressingon a “generate image” button of the scanner for each image).Alternatively, a result of the intraoral scan may be one or more videosof the patient's oral cavity. An operator may start recording the videowith the scanner 150 at a first position in the oral cavity, move thescanner 150 within the oral cavity to a second position while the videois being taken, and then stop recording the video. The scanner 150 maytransmit the discrete intraoral images or intraoral video (referred tocollectively as image data) to the computing device 105. Note that asused herein image data may be actual two-dimensional orthree-dimensional images (e.g., discrete intraoral images or intraoralvideo), a representation of a dental arch (e.g., a virtualthree-dimensional model of the dental arch), an x-ray image, a computedtomography (CT) image, or a combination thereof. Accordingly, the termimage data may not actually include images in some embodiments.Computing device 105 may store the image data in data store 110. Theimage data may include past image data 132 generated by a scanner 150 ata first time and current image data 135 generated by the scanner 150 oran additional scanner at a second later time. Alternatively, scanner 150may be connected to another system that stores the past image data 132and/or current image data 135 in data store 110. In such an embodiment,scanner 150 may not be connected to computing device 105.

According to an example, a user (e.g., a practitioner) may subject apatient to intraoral scanning. In doing so, the user may apply scanner150 to one or more patient intraoral locations. The scanning may bedivided into one or more segments. As an example the segments mayinclude a lower buccal region of the patient, a lower lingual region ofthe patient, a upper buccal region of the patient, an upper lingualregion of the patient, one or more preparation teeth of the patient(e.g., teeth of the patient to which a dental device such as a crown oran orthodontic alignment device will be applied), one or more teethwhich are contacts of preparation teeth (e.g., teeth not themselvessubject to a dental device but which are located next to one or moresuch teeth or which interface with one or more such teeth upon mouthclosure), and/or patient bite (e.g., scanning performed with closure ofthe patient's mouth with scan being directed towards an interface areaof the patient's upper and lower teeth). Via such scanner application,the scanner 150 may provide current image data (also referred to as scandata) 135 to computing device 105. The current image data 135 mayinclude 2D intraoral images and/or 3D intraoral images.

The current image data 135 and past image data 132 may each be used togenerate a virtual model (e.g., a virtual 2D model or virtual 3D model)of the patient's dental arch in some embodiments. Each virtual model mayreflect the condition of the dental arch at a particular point in time.Each virtual model may include a 3D surface and appearance propertiesmapped to each point of the 3D surface (e.g., a color of the surface ateach point). To generate a virtual model, intraoral scan application 108may register (i.e., “stitch” together) the intraoral images generatedfrom an intraoral scan session. In one embodiment, performing imageregistration includes capturing 3D data of various points of a surfacein multiple images (views from a camera), and registering the images bycomputing transformations between the images. The images may then beintegrated into a common reference frame by applying appropriatetransformations to points of each registered image.

In one embodiment, image registration is performed for each pair ofadjacent or overlapping intraoral images (e.g., each successive frame ofan intraoral video) generated during an intraoral scan session. Imageregistration algorithms are carried out to register two adjacentintraoral images, which essentially involves determination of thetransformations which align one image with the other. Image registrationmay involve identifying multiple points in each image (e.g., pointclouds) of an image pair, surface fitting to the points of each image,and using local searches around points to match points of the twoadjacent images. For example, intraoral scan application 108 may matchpoints of one image with the closest points interpolated on the surfaceof the other image, and iteratively minimize the distance betweenmatched points. Intraoral scan application 108 may also find the bestmatch of curvature features at points of one image with curvaturefeatures at points interpolated on the surface of the other image,without iteration. Intraoral scan application 108 may also find the bestmatch of spin-image point features at points of one image withspin-image point features at points interpolated on the surface of theother image, without iteration. Other techniques that may be used forimage registration include those based on determining point-to-pointcorrespondences using other features and minimization ofpoint-to-surface distances, for example. Other image registrationtechniques may also be used.

Many image registration algorithms perform the fitting of a surface tothe points in adjacent images, which can be done in numerous ways.Parametric surfaces such as Bezier and B-Spline surfaces are mostcommon, although others may be used. A single surface patch may be fitto all points of an image, or alternatively, separate surface patchesmay be fit to any number of a subset of points of the image. Separatesurface patches may be fit to have common boundaries or they may be fitto overlap. Surfaces or surface patches may be fit to interpolatemultiple points by using a control-point net having the same number ofpoints as a grid of points being fit, or the surface may approximate thepoints by using a control-point net which has fewer number of controlpoints than the grid of points being fit. Various matching techniquesmay also be employed by the image registration algorithms.

In one embodiment, intraoral scan application 108 may determine a pointmatch between images, which may take the form of a two dimensional (2D)curvature array. A local search for a matching point feature in acorresponding surface patch of an adjacent image is carried out bycomputing features at points sampled in a region surrounding theparametrically similar point. Once corresponding point sets aredetermined between surface patches of the two images, determination ofthe transformation between the two sets of corresponding points in twocoordinate frames can be solved. Essentially, an image registrationalgorithm may compute a transformation between two adjacent images thatwill minimize the distances between points on one surface, and theclosest points to them found in the interpolated region on the otherimage surface used as a reference.

Intraoral scan application 108 repeats image registration for alladjacent image pairs of a sequence of intraoral images to obtain atransformation between each pair of images, to register each image withthe previous one. Intraoral scan application 108 then integrates allimages into a single virtual 3D model by applying the appropriatedetermined transformations to each of the images. Each transformationmay include a rigid body motion (e.g., rotations and/or translations).

In addition to current image data 135 and past image data 132, imagedata to be compared may additionally include multiple differentinstances of past image data 132. For example, image data may includefirst past image data based on a first intraoral scan taken at a firsttime, second past image data based on a second intraoral scan taken at asecond time, and so on. Comparisons may be made between each of theimage data. Alternatively, comparisons may be made between adjacent intime image data. For example, first past image data may be compared tosecond past image data and second past image data may additionally becompared to current image data in the above example. Each of the currentimage data 135 and past image data 132 may include additional collectedinformation such as the time that the image data was generated (e.g.,the time that a scan was performed), clinical verbal information (e.g.,specific patient complaints at a time of scanning), scanner or modelcreation information (e.g., a type of scanner used to generate imagesfrom which a virtual 3D model was generated), and so on.

In addition to current image data 135 and past image data 132 includingdata captured by scanner 150 and/or data generated from such captureddata (e.g., a virtual 3D model), image data may also include data fromone or more additional image capture devices 160. The additional imagecapture devices 160 may include an x-ray device capable of generatingstandard x-rays (e.g., bite wing x-rays), panoramic x-rays,cephalometric x-rays, and so on. The additional image capture devices160 may additionally or alternatively include an x-ray device capable ofgenerating a cone beam computed tomography (CBCT) scan. Additionally, oralternatively, the additional image capture devices 160 may include astandard optical image capture device (e.g., a camera) that generatestwo-dimensional or three-dimensional images or videos of a patient'soral cavity and dental arch. For example, the additional image capturedevice 160 may be a mobile phone, a laptop computer, an image captureaccessory attached to a laptop or desktop computer (e.g., a device thatuses Intel® RealSense™ 3D image capture technology), and so on. Such anadditional image capture device 160 may be operated by a patient or afriend or family of the patient, and may generate 2D or 3D images thatare sent to the computing device 105 via network 170. Additionally, anadditional image capture device 160 may include an infrared (IR) camerathat generates near IR images. Accordingly, current image data 135 andpast image data 132 may include 2D optical images, 3D optical images,virtual 2D models, virtual 3D models, 2D x-ray images, 3D x-ray images,and so on.

Dental issue identifier 115 compares current image data 135 of a dentalarch to past image data 132 of the dental arch to identify changes thathave occurred to the dental arch. However, detected differences includeboth differences caused by scanner inaccuracy as well as differencescaused by clinical changes such as tooth wear (also referred to as tootherosion), gum recession, gum swelling, occlusion (e.g., bite surfaces)and so on. Dental issue identifier 115 identifies those differencescaused by scanner inaccuracy and filters them out. Additionally, dentalissue identifier 115 may apply a low pass filter to smooth out very highnoise-like errors (e.g., to smooth out errors having a frequency ofhigher than 100 microns). The remaining differences that are caused byclinical changes to the dental arch may then be identified, classifiedand displayed. The dental issue identifier 115 is discussed in greaterdetail below with reference to FIG. 1B.

In embodiments, a first intraoral scan that produces past image data 132is performed at the start of orthodontic treatment, and a secondintraoral scan that produces the current image data 135 is performedduring the orthodontic treatment (e.g., during an intermediate stage ofa multi-stage orthodontic treatment plan). Alternatively, oradditionally, other image data may be generated during at the start ofthe orthodontic treatment and during the orthodontic treatment.Moreover, multiple scans may be performed during the orthodontictreatment.

The task of identifying changes of clinical significance is made morecomplex during an orthodontic treatment. For such an orthodontictreatment, teeth may be moved according to a treatment plan and at thesame time unplanned tooth wear, gum recession, tooth chips, and so onmay occur. The planned tooth changes may occlude the unplanned changesof clinical significance in some instances. Additionally, other types oftreatments may also increase the complexity of identifying changes ofclinical significance. For example, a restorative treatment may causeone or more teeth to change their shape, which may occlude unplannedchanges. Additionally, in a hygienist treatment some tartar may beremoved, which may change interproximal tooth regions as well as somegum regions. These differences caused by the hygienist treatment (e.g.,tooth cleaning) or other types of treatment can be taken into account todetermine whether a change is of clinical significance. For example,information such as a date of a hygienist treatment or a date and/ortooth shape of a restorative treatment may be used to help classify achange as a clinical change or non-clinical change.

A multi-stage orthodontic treatment plan may be for a multi-stageorthodontic treatment or procedure. The term orthodontic procedurerefers, inter alia, to any procedure involving the oral cavity anddirected to the design, manufacture or installation of orthodonticelements at a dental site within the oral cavity, or a real or virtualmodel thereof, or directed to the design and preparation of the dentalsite to receive such orthodontic elements. These elements may beappliances including but not limited to brackets and wires, retainers,aligners, or functional appliances. Different aligners may be formed foreach treatment stage to provide forces to move the patient's teeth. Theshape of each aligner is unique and customized for a particular patientand a particular treatment stage. The aligners each have teeth-receivingcavities that receive and resiliently reposition the teeth in accordancewith a particular treatment stage.

The multi-stage orthodontic treatment plan for a patient may haveinitially been generated by a dental practitioner (e.g., anorthodontist) after performing a scan of an initial pre-treatmentcondition of the patient's dental arch, which may be represented in thepast image data 132. The treatment plan may also begin at home (based ona patient scan of himself) or at a scanning center. The treatment planmight be created automatically or by a professional (including anOrthodontist) in a remote service center. The scan may provide 3Dsurface data (e.g., surface topography data) for the patient's intraoralcavity (including teeth, gingival tissues, etc.). The 3D surface datacan be generated by directly scanning the intraoral cavity, a physicalmodel (positive or negative) of the intraoral cavity, or an impressionof the intraoral cavity, using a suitable scanning device (e.g., ahandheld scanner, desktop scanner, etc.). Image data from the initialintraoral scan may be used to generate a virtual three-dimensional (3D)model or other digital representation of the initial or startingcondition for the patient's upper and/or lower dental arches.

The dental practitioner may then determine a desired final condition forthe patient's dental arch. The final condition of the patient's dentalarch may include a final arrangement, position, orientation, etc. of thepatient's teeth, and may additionally include a final bite position, afinal occlusion surface, a final arch length, and so on. A movement pathof some or all of the patient's teeth and the patient bite changes fromstarting positions to planned final positions may then be calculated. Inmany embodiments, the movement path is calculated using one or moresuitable computer programs, which can take digital representations ofthe initial and final positions as input, and provide a digitalrepresentation of the movement path as output. The movement path for anygiven tooth may be calculated based on the positions and/or movementpaths of other teeth in the patient's dentition. For example, themovement path can be optimized based on minimizing the total distancemoved, preventing collisions between teeth, avoiding tooth movementsthat are more difficult to achieve, or any other suitable criteria. Insome instances, the movement path can be provided as a series ofincremental tooth movements that, when performed in sequence, result inrepositioning of patient's teeth from the starting positions to thefinal positions.

Multiple treatment stages may then be generated based on the determinedmovement path. Each of the treatment stages can be incrementalrepositioning stages of an orthodontic treatment procedure designed tomove one or more of the patient's teeth from a starting tootharrangement for that treatment stage to a target arrangement for thattreatment stage. One or a set of orthodontic appliances (e.g., aligners)are then fabricated based on the generated treatment stages (e.g., basedon the virtual 3D models of the target conditions for each of thetreatment stages). For example, a set of appliances can be fabricated,each shaped to accommodate a tooth arrangement specified by one of thetreatment stages, such that the appliances can be sequentially worn bythe patient to incrementally reposition the teeth from the initialarrangement to the target arrangement. The configuration of the alignerscan be selected to elicit the tooth movements specified by thecorresponding treatment stage.

The current image data 135 received during an intermediate stage in themulti-stage orthodontic treatment plan may be compared by dental issueidentifier 115 to the past image data 132. Based on the comparison,dental issue identifier 115 determines clinical changes that haveoccurred to the dental arch, and compares those clinical changes thatare detected to expected clinical changes that are specified in theorthodontic treatment plan. Any deviation between the actual conditionof the patient's dental arch and the planned condition of the patient'sdental arch for the current treatment stage may then be determined. Thedental practitioner may then take one or more corrective actions basedon the detected deviation.

In some embodiments, current image data 135 received during anintermediate stage in a multi-stage orthodontic treatment plan may beused to analyze a fit of a next aligner based on the actual currentcondition of the dental arch (e.g., based on current teeth positions,occlusion, arch width, and so on). If the next aligner will not have anoptimal fit on the patient's dental arch (e.g., will not fit onto thedental arch or will fit but will not apply the desired forces on one ormore teeth), then new aligners may be designed based on updating thetreatment plan staging.

FIG. 1B illustrates one embodiment of a dental issue identifier 115, inaccordance with an embodiment. In one embodiment, the dental issueidentifier 115 includes a spatial comparator 168, an appearancecomparator 172, an image difference separator 178, a representationgenerator 179 and a display module 118. Alternatively, one or more ofthe spatial comparator 168, appearance comparator 172, image differenceseparator 178, representation generator 179 and/or display module 118may be combined into a single module or further divided into additionalmodules.

Spatial comparator 168 compares spatial information from first imagedata 162 with spatial information from second image data 163. The firstimage data 162 may have been generated from an intraoral scan taken at afirst time and may be considered a reference surface. The second imagedata 163 may have been generated from an intraoral scan taken at asecond time and may be considered a test surface. The first image data162 may be or include a first virtual 3D model of the dental arch thatrepresents a condition of the dental arch at the first time, and thesecond image data 163 may be or include a second virtual 3D model of thedental arch that represents a condition of the dental arch at the secondtime. A representation of the dental arch (e.g., a first 3D virtualmodel) in the first image data 162 may be compared with a representationof the dental arch (e.g., a second virtual 3D model) in the second imagedata 163.

Spatial comparison of the first image data 162 with the second imagedata 163 may include performing image registration between the firstimage data 162 and second image data 163. The image registrationinvolves determination of the transformations which align one image withthe other. Image registration may involve identifying multiple points,point clouds, edges, corners, surface vectors, etc. in each image of animage pair, surface fitting to the points of each image, and using localsearches around points to match points of the two images. For example,spatial comparator 168 may match points of one image with the closestpoints interpolated on the surface of the other image, and iterativelyminimize the distance between matched points. Spatial comparator 168 mayselect the points based on a random sampling of surface vertices, basedon binning of vertices to a voxel grid and averaging each voxel, basedon feature detection (e.g., detecting tooth cusps), and/or based onother techniques. Spatial comparator 168 may also find the best match ofcurvature features at points of one image with curvature features atpoints interpolated on the surface of the other image, with or withoutiteration. Spatial comparator 168 may also find the best match ofspin-image point features at points of one image with spin-image pointfeatures at points interpolated on the surface of the other image, withor without iteration. Other techniques that may be used for imageregistration include those based on determining point-to-pointcorrespondences using other features and minimization ofpoint-to-surface distances, for example. Other image registrationtechniques may also be used.

Many image registration algorithms perform the fitting of a surface tothe points in adjacent images, which can be done in numerous ways.Parametric surfaces such as Bezier and B-Spline surfaces are common,although others may be used. A single surface patch may be fit to allpoints of an image, or alternatively, separate surface patches may befit to any number of a subset of points of the image. Separate surfacepatches may be fit to have common boundaries or they may be fit tooverlap. Surfaces or surface patches may be fit to interpolate multiplepoints by using a control-point net having the same number of points asa grid of points being fit, or the surface may approximate the points byusing a control-point net which has fewer number of control points thanthe grid of points being fit. Surface patches may be selected usingvarious techniques, such as by selecting parts of a surface that areless than a threshold distance (e.g., in millimeters) away from aselected point (where a connected component is generated that includesthe point itself), by performing tooth segmentation and associatingpoints with a tooth crown that includes a selected point or a part ofsuch a crown, and so on. Various matching techniques may also beemployed by the image registration algorithms.

In one embodiment, spatial comparator 168 may determine a point matchbetween images, which may take the form of a two dimensional (2D)curvature array. A local search for a matching point feature in acorresponding surface patch of another image is carried out by computingfeatures at points sampled in a region surrounding the parametricallysimilar point. One matching technique that may be used includes runningan iterative closest point (ICP) algorithm from several staringpositions. Another matching technique includes detecting a number oforientation-independent surface features on test surfaces of one imageand reference surfaces on the other image. For each feature on a testsurface, spatial comparator 168 may find all similar features on thereference surface and vote for particular transforms that would alignfeatures on the test surface with the features on the reference surface.The transformation with the most votes may then be picked, and a resultmay be refined using an ICP algorithm.

Spatial comparator 168 may validate the quality of detected matches witha suitable method and discard points that did not match well. Onesuitable method for validation includes computing a percentage ofsurface area on a tested part of the test surface that is less than athreshold distance (e.g., in millimeters) away from the referencesurface after alignment. A match may be validated if the size of thesurface area that matched is larger than a size threshold. Anothersuitable method for validation includes computing an average or mediandistance between vertices of a tested part of the test surface and thereference surface after alignment. If the average or median betweenvertices is less than a threshold distance, then validation may besuccessful.

Spatial comparator 168 may compute a mean and/or median alignment of theentire set of matches. Spatial comparator 168 may detect and removeoutliers that suggest variants of alignment of the test surface andreference surface that are too different from the mean or medianalignment of the entire set of matches by using an appropriate method.For example, surface patches for which the alignment of the test surfaceto the reference surface has an alignment value that differs from themean or median by more than a threshold may be too different. Oneappropriate method that may be used is the random sample consensus(RANSAC) algorithm. If two surfaces are not comparable, then spatialcomparator 168 will determine that the registration has failed becausethe number of surviving points would be too small to reasonably coverthe entire test surface. This might happen, for example, if input datacontained a mistake (e.g., a user tried to match an intraoral scan ofone person to an intraoral scan of another person). Spatial comparator168 may check for such a condition and report an error if this occurs.

A result of the surface matching may be a dense set of pairs of matchingpoints, with each pair corresponding to a region on the test surface anda matching region on the reference surface. Each such region is alsoassociated with a point, so each pair can also be viewed as a pair of apoint on the test surface and a matching point on reference surface. Anordered set of these points on a test surface is a point cloud on thetest surface, and a set of matching points on a reference surfaceordered in the same way is a matching point cloud on the referencesurface.

A suitable algorithm may be used to compute an approximate alignment ofa test point cloud to a reference point cloud. One example of such asuitable algorithm includes the least-squares minimization of distancebetween test and reference point clouds. After approximate alignment viaa rigid transformation of the test surface, test and reference pointclouds won't coincide exactly. A suitable algorithm may be used tocompute a non-rigid transformation such as a piecewise-smooth warp spacetransformation that smoothly deforms a 3D space such that 1) thisdeformation is as smooth as possible and b) the test point cloud afterapplication of the warp transformation is much better aligned with thereference point cloud. Possible implementation options include, but arenot limited to, radial basis function interpolation, thin-plate splines(TPS) and estimating teeth movements and propagating them to a nearbyspace.

Accordingly, once corresponding point sets are determined betweensurface patches of the two images, determination of the transformationbetween the two sets of corresponding points in two coordinate framescan be solved. Essentially, an image registration algorithm may computea transformation between two images that will minimize the distancesbetween points on one surface, and the closest points to them found inthe interpolated region on the other image surface can be used as areference. The transformation may include rotations and/or translationalmovement in up to six degrees of freedom. Additionally, thetransformation may include deformation of one or both of the images(e.g., warp space transformations and/or other non-rigidtransformations). A result of the image registration may be one or moretransformation matrix that indicates the rotations, translations and/ordeformations that will cause the one image to correspond to the otherimage.

A result of the spatial comparison performed by spatial comparator 186may include an alignment transformation (e.g., rigid transformation inposition and/or orientation to achieve a rigid body alignment) and awarp space transformation or other non-rigid transformation (e.g.,smooth deformation of 3D space). Image difference separator 178 may usesuch information to determine differences between points, point clouds,features, etc. on a first representation of the dental arch (referencesurface) from the first image data 162 and a second representation ofthe dental arch (test surface) from the second image data 163. Imagedifference separator 178 may then distinguish between differences thatare attributable to scanner inaccuracy and differences that areattributable to clinical changes in the dental arch.

Differences between the first representation of the dental arch and thesecond representation of the dental arch generally occur in differentspatial frequency domains. For example, relatively smooth changes thatoccur in arch length and jaw width are generally caused by scannerinaccuracy and happen with a low spatial frequency and a high magnitude.For example, the human jaw typically does not change in width over timefor an adult. However, for intraoral scanners with a limited FOV, it iscommon for the measured jaw or arch width (distance between last molarsof the jaw) to vary between scans. Other types of changes that haveclinical significance generally occur with a much higher spatialfrequency and a much lower magnitude. As used herein, spatial frequencymeans changes in a vector field with lateral changes in position, whereeach vector in the vector field represents where a point of the testsurface would move after applying the rigid and non-rigidtransformations. Differences between the first representation and thesecond representation that change slowly with lateral changes inposition have a low spatial frequency or lateral frequency, and areglobal differences that may affect the entire dental arch or a largeregion of the dental arch. These would show as a smooth vector field,where the vectors of points have similar values to vectors of nearbypoints.

Scanner errors may also have a very high spatial frequency or a veryfine scale. This may be noise introduced by the scanner, and may befiltered out with a low pass filter. Spatial differences with a veryhigh spatial frequency (a very small scale) may be those spatialdifferences with a spatial frequency that exceeds a frequency threshold.Spatial differences attributable to clinical changes may have a spatialfrequency that is lower than the frequency threshold.

Changes of clinical significance are local changes that occur only in asmall region on the dental arch. Changes of tooth wear happen on a scaleof a small tip of a tooth, and occur only at that region. Accordingly,tooth wear has a high spatial frequency. Additionally, the magnitude oftooth wear is generally small (e.g., fractions of a millimeter).Similarly, changes in gum line (e.g., gum recession) happen at the gumline and in a particular direction (perpendicular to the gum line).Additionally, the magnitude of gum recession and gum swelling can alsobe quite small (e.g., fractions of a millimeter). Tooth movement happensat the scale of the tooth size. The known scales of each of these typesof clinical changes can be used to separate differences between dentalarch representations caused by scanner inaccuracy and clinical changes,and can be further used to separate out and/or classify each of thedifferent types of clinical changes.

To separate the differences between representations of the dental archthat are based on scanner inaccuracy, the smooth or low frequencychanges can be filtered out, such as by applying a low pass filter. Suchsmooth changes can be identified by the low pass filter and then removedfrom the test surface (representation of the dental arch from the secondimage data). In other words, this error or difference may be subtractedfrom the test surface for a better fit between the test surface andreference surface. The low pass filter may filter out spatialdifferences having a spatial frequency that is higher than a frequencythreshold.

In one embodiment, alignment transformation (e.g., transformation inposition and/or orientation) and warp space transformation (e.g.,deformation) computed from a point cloud may be applied to the entiretest surface. A magnitude of the warp space transformation may bereported as a “global” component of a surface difference and visualizedwith a suitable method. Representation generator 179 may generate arepresentation of the aligned and warped second image data (e.g., athird representation of the dental arch that is computed based onapplying the alignment transformation and the warp space transformationto a second representation of the dental arch from the second image data163). Suitable methods include a per-point coloring of the test surfaceaccording to a magnitude of the warp movement at this point and coloringof an entire tooth crown according to a magnitude of its movement. Suchglobal component of the surface difference may be the surface differencethat is attributable to scanner inaccuracy.

In one embodiment, a suitable algorithm may be used to compute residualdifferences between the aligned and warped test surface and thereference surface. This residual may be reported as a “local” componentof the surface difference. A possible implementation option includescomputing a distance to a nearest point of reference surface for eachpoint of an aligned and warped test surface and coloring the testsurface accordingly (e.g., coloring based on a magnitude of thedistance, where a first color may be used for a first distancemagnitude, a second color may be used for a second distance magnitude,and so on). Another possible implementation option includes performingan additional comparison as set forth above to split the remainingdifferences into additional levels (e.g., into tooth movement, changesin gingival shape, changes to the tooth surface, changes in toothcrowding, changes in tooth spacing, occlusion changes, orthodonticrelapse, changes to proximal contacts, and so on).

To split the differences into different levels, one or more filters orfiltering operations may be applied to separate the differences intodifferences of different spatial frequencies or scale. These filters maybe applied based on the registration information output by spatialcomparator 168 as described above. Alternatively, or additionally, imagedifference separator 178 may invoke spatial comparator 168 to repeat theabove described operations with regards to image comparison andregistration using the third representation previously output by thespatial comparator 168 as a starting point rather than using the secondimage data 163 as the starting point. One or more filters such as lowpass filters may then be applied to distinguish each level of localchanges.

Each of the applied filters may be low pass filters that look fordifferences in surfaces at points that are “similar” to differences atnearby points (e.g., by performing approximate rigid bodytransformation, by fitting a smooth deformation function to test data,or by segmenting a virtual 3D model into teeth and computing motion ofeach tooth separately). Spatial comparator 168 and/or differenceseparator 178 may compute a “low-frequency” component, then effectively“subtract” that low-frequency component from the surface or otherwisecompensate for the computed low-frequency differences between the targetsurface and the reference surface. Subtraction of the low-frequencycomponent from the data may effectively form a complimentary high-passfilter. This process may be performed repeatedly, with each applicationof the process detecting a lowest frequency component that has not yetbeen filtered out.

For example, a first low pass filter may be applied to filter out alldifferences attributable to scanner inaccuracies that cause a detecteddifference in arch length or jaw width. Then another low pass filter maybe applied to this already filtered data. With some lower-frequencycomponents already filtered out, the net result is the same as if abandpass filter for a specific range of spatial frequencies was appliedto original data. For example, subtraction of the error component yieldsdata that corresponds to application of a high-pass filter with highercutoff frequency to the original surface. This process can be repeatedas many times as desired to generate a sequence of differences orchanges corresponding to different bands of spatial frequencies. Forexample, a second low pass filter may be applied to filter out alldifferences attributable to tooth movement. A third low pass filter maythen be applied to filter out all differences attributable to gumrecession or tooth wear, and so on. Accordingly, a chained sequence offilters may be applied, where output of a low-pass filter is fed as aninput to another low-pass filter with a higher spatial frequency cutoff.

Embodiments split the data into non-overlapping bands of frequencies.Differences may be classified based on their scale or spatial frequency.For example, the spatial differences may be classified into a toothmovement classification, a tooth wear classification, a gum recessionclassification and/or a gum swelling classification.

Another technique that may be used to separate the differences intodifferent classifications or categories is applying specific rules oralgorithms to the first representation of the dental arch in the firstimage data and the third representation of the dental arch that isoutput by the spatial comparator 168. Each rule or algorithm may beconfigured to detect a particular type of clinical change. One or moreof the rules or algorithms may detect specific changes based on asegmentation of the teeth and gums. Accordingly, the following examplesmay be performed after segmentation has been performed for the teeth andgums. For example, a gum recession detection rule may apply featuredetection techniques to identify the gum line and the teeth in thedental arch in both representations. The gum recession detection rulemay then measure a minimal distance between the gum line and peaks ofeach of the teeth in both representations. These minimal distancemeasurements may then be compared between the representations. If, forexample, the detected distance between the gum line and the peak oftooth 1 is greater in the third representation of the dental arch thanin the first representation of the dental arch, then gum recession isidentified. The distance measured in the first representation may besubtracted from the distance in the third representation to measure theactual amount of gum recession. A rate of recession may then bedetermined by determining the amount of time that elapsed betweengeneration of the first image data 162 and the second image data 163,and the distance may be divided by the elapsed time to compute a rate ofthe gum recession.

In another example, a tooth movement detection rule may detect toothmovement. This may include first performing feature detection techniquesto identify individual teeth. For each tooth, a change in position andorientation of the tooth between the first representation and the thirdrepresentation may be determined based on the performed imageregistration. The change in position and orientation may be divided intodifferent types of tooth movement, including lateral movement alongdifferent axes, movement into the jaw or away from the jaw, rotationsabout various axes, and so on. Each type of tooth movement may bemeasured and displayed. Additionally, a rate of movement may bedetermined based on dividing the magnitude of the movement by theelapsed time between the taking of the first image data and the takingof the second image data.

In some instances, the second image data 163 may be generated at anintermediate stage in orthodontic treatment. In such instances, theremay be planned movement for various teeth. The actual detected toothmovement may be compared to the planned tooth movement to determine ifthe treatment is progressing as planned. Various thresholds may beapplied to the tooth movements to determine this information.

FIG. 11 illustrates a chart 1100 showing different types of toothmovement. A treatment plan may call for a particular amount of toothmovement between an initial position 1105 and a planned position 1110 atan intermediate treatment stage. For translational movement the plannedmovement may be reflected in terms of distance (e.g., mm or inches) andfor rotational movement the planned movement may be reflected in termsof degrees of rotation. Translational movement can include movement in aplane defined by the arch (e.g., movement along the arch that affectsthe separation between teeth) and can additionally or alternativelyinclude movement outside of the plane (e.g., raising a tooth or loweringa tooth). If the actual tooth position deviates from the initialposition 1105 by less than a first threshold 1115 (compliancethreshold), then it may be determined that the tooth has not changedposition. It may be determined that the tooth has not moved if the toothmovement is within the first threshold 1115 in the planned direction ofmovement and/or in an opposite direction of movement from the planneddirection of movement. If tooth movement was less than planned and theactual tooth position exceeds the first threshold 1115 but is less thana second threshold 1120, then it may be determined that the toothmovement was less than planned. Alternatively, or additionally, thethreshold may be based on a difference between a planned movement and anactual movement (e.g., if actual tooth position deviated from plannedtooth position by more than a threshold amount, then it may bedetermined that the tooth movement was less than planned. If thedetected movement exceeds the second threshold 1120, then tooth movementmay be normal. Alternatively, or additionally, if the actual toothmovement deviates from the planned tooth movement by less than athreshold, then the tooth movement may be identified as normal. If theactual position shows movement in an opposite direction from the plannedmovement and exceeds the first threshold 1115, then it may be determinedthat tooth movement opposite to what was planned has been achieved.

FIG. 12 illustrates a table showing which teeth have positions that arein compliance with an orthodontic treatment plan and which teeth havepositions that deviate from the orthodontic treatment plan, inaccordance with an embodiment. As shown, the table includes a differentrow for each tooth, and different columns for each type of movement thatis measured for the teeth. The tooth rows may be labeled by tooth numberor tooth ID. The different types of tooth motion that are measuredinclude bucco-lingual translational motion, mesio-distal translationalmotion, intrusion/extrusion tooth motion, angulation, inclination, androtation. Angulation, inclination and rotation may each be considered asrotation about a different axis, and may be measured in degrees orrotation. The table shows the achieved tooth motion and planned toothmotion for each type of tooth motion. Additionally, the table includesan alert column that indicates any clinical signs that have beenidentified.

Computed difference data from several levels can be representedseparately for each level or combined to show several levels at once inthe third representation generated by representation generator 179. Thisdata can also be filtered to hide insignificant changes and simplifyanalysis by highlighting regions of significant change.

Returning to FIG. 1B, representation generator 179 may generate avirtual 2D or 3D model of the patient's dental arch, or may generate avisualization of a virtual 2D or 3D model output by spatial comparator168 and/or image difference separator 178. For example, representationgenerator 179 may generate a modified version of the thirdrepresentation generated by the spatial comparator 168 or imagedifference separator 178. Alternatively, representation generator 179may generate a generic image of a dental arch. An overlay generator 182of the representation generator 179 may generate a graphical overlay(e.g., a color overlay) that marks regions of the dental arch for whichimage differences associated with clinical changes have been identified.For example, a first color (e.g., purple) may be used to indicate teethassociated with tooth movement opposite to what was planned, a secondcolor (e.g., yellow) may be used to indicate teeth associated with toothmovement that is less than planned, a third color (e.g., green) may beused to indicate teeth that have moved as planned, and a fourth color(e.g., blue) may be used to indicate teeth associated with toothmovement that is greater than planned. The graphical overlay may alsocolor in regions of the gums where recession has occurred with aparticular color, regions of the gums where swelling has occurred withanother color, teeth (or regions of teeth) that have been worn down withanother color, and so on.

FIG. 13 illustrates a virtual model of a patient's dental arch 1300 inwhich the teeth are color coded (or coded based on fill pattern) basedon a detected clinical change that was detected for those teeth (or lackthereof) between current image data and past image data. As shown, teeththat have moved as planned 1315 are shown with a first color and/orother marking (e.g., green and/or a first hatching) and teeth that havemoved less than planned are shown with a second color and/or othermarking (e.g., orange or yellow and/or a second hatching). Additionally,gums 1320 may be shown with a third color and/or other marking (e.g.,red).

Other types of labels or flags may be used to present and/or call outthe various clinical signs, such as numerical labels, flags with textlabels, and so on. These additional types of labels and/or flags may beused instead of or in addition to a color overlay or other visualoverlay.

FIG. 14A illustrates a comparison between two different scans of adental arch, in accordance with an embodiment. As shown, a first scan1405 is compared to a second scan 1410 in the manner described above. Asa result of image registration, a past condition of a tooth 1425 iscompared to a current condition of the tooth 1430. Based on thecomparison, a representation of the dental arch 1415 that includes thetooth is generated. The representation of the tooth includes ahighlighted or accentuated region 1435 that shows an amount of the tooththat has been worn away between the first scan 1405 and the second scan1410.

Referring back to FIG. 1B, representation generator 179 may include animage difference extrapolator 184. Image difference extrapolator 184 maycompute a magnitude of a difference between the first representation ofthe dental arch from the first image data 162 and a third representationof the dental arch as generated based on a comparison between the firstimage data 162 and the second image data 163. In one embodiment, imagedifference extrapolator 184 applies one or more color filters (and/orother appearance filters) to normalize aspects of the appearance betweenrepresentations of the dental arch due to scanner differences.

As indicated above, first image data 162 and second image data 163 mayeach have a time stamp. Additionally, further image data may also becompared against the first image data and/or second image data and mayalso have time stamps. Image difference extrapolator 184 additionallydetermines an amount of time that separates the first image data 162 andthe second image data 163. Additionally, image difference extrapolator184 may determine amounts of time that separate the first image data 162and second image data 163 from the further image data and/or amounts oftime that separate the further image data from each other. Based on thisinformation, image difference extrapolator 184 determines a rate ofchange associated with the difference. Image difference extrapolator 184may apply the rate of change to the third representation to determine apredicted change to a region of the dental arch at a future date.Representation generator 179 may then generate a new representation ofthe dental arch that includes extrapolated changes to regions of thedental arch that are computed by image difference extrapolator. Thisrepresentation may accentuate and exaggerate the changes of clinicalsignificance to call them to the attention of the dental practitioner.

Additionally, after determining a rate of chance, image differenceextrapolator 184 may estimate a time when a sub-clinical issue mayevolve into a problem (into an issue of clinical significance). Forexample, image difference extrapolator 184 may compare a clinical changeto a severity threshold. If the clinical change is below the severitythreshold, then it may represent a sub-clinical issue and may not besignificant enough to treat presently. However, over time thatsub-clinical issue may develop in to a clinical issue. Accordingly, therate of change may be applied to determine a future time at which thesub-clinical issue might become a clinical issue. Additional image datamay be taken at the future time to determine whether the sub-clinicalissue has developed into a clinical issue.

In an example, assume that a tooth wear problem is detected with a wearrate of 0.1 mm per month. The wear rate may not be critical yet, but itmay present a problem in a year after 1.2 mm of enamel has been wornoff. Alternatively, the identified wear rate of 0.1 mm per month may bea false alarm. Without additional data points, it may be difficult todistinguish between a false alarm and a clinical issue. Accordingly,image difference extrapolator 184 may determine a recommended futuretime to generate next image data and re-evaluate the possible tooth wearproblem. Additional image data may be generated at the recommendedfuture time, and the above discussed operations may be performed todetermine clinical changes between the additional image data and thesecond image data 163. If the tooth wear in the above example hascontinued, then the sub-clinical issue may be identified as a clinicalissue. If the tooth wear has not continued, then the sub-clinical issuemay be identified as a false alarm.

If the further or additional image data is used (e.g., more than justthe first image data 162 and second image data 163), then adetermination may be made as to whether the rate of change for the oneor more of the changes of clinical significance is accelerating ordecelerating. This may be used to more accurately predict a futurecondition of clinical significance.

FIG. 14B illustrates a representation 1450 of a dental arch showingchanges that have occurred to the dental arch over time, in accordancewith an embodiment. FIB. 14B shows the representation of the dental arch1415 of FIG. 14A, along with additional information. As shown, inaddition to showing the highlighted or accentuated region 1435 thatidentifies an amount of the tooth that has been worn away between thefirst scan 1405 and the second scan 1410, the representation 1450 alsoincludes flag 1455 that points to the accentuated region 1435 and anadditional flag 1460 that points to a region of gum swelling.Additionally, the representation 1450 is shown with a timeline thatshows various past amounts of wear for the tooth and a predicted futureamount of wear for the tooth. The timeline also includes a suggestedtreatment option of a night guard to reduce tooth wear.

Referring back to FIG. 1B, first image data 162 and second image data163 may include bite information such as an occlusion surface. Occlusionrefers to the contact between teeth. More particularly, occlusion is therelationship between the maxillary (upper) teeth and the mandibular(lower) teeth, such as during chewing. A malocclusion is themisalignment of the teeth and/or jaw that impairs a person's bite.Contacts between maxillary teeth and mandibular teeth may be dividedinto functional contacts and interfering contacts. The first image data162 and/or second image data may include an occlusion map that showsfunctional contacts and interfering contacts. These functional contactsand/or interfering contacts may change over time. For example, a patientmay develop further interfering contacts over time. This change inocclusion may be a change of clinical significance that is determinedusing the techniques set forth above.

Display module 118 may output the representations and/or visual overlaysgenerated by other modules for display to a user.

In addition to spatial changes in the dental arch such as toothmovement, gum recession, tooth wear, tooth occlusion, and gum swelling,appearance changes can also be clinically significant. For example, achange in color of a tooth or tooth region can point to variousproblems, such as development of a lesion or dental caries. For example,a dark spot or a white spot may indicate a problem area in a tooth. Alesion can start as a very small color change, and grow to a larger dot,before penetrating into inner layers of a tooth. However, some toothstains may look like lesions. Comparing appearance of the dental archover time (in a longitudinal study) can help early detection of lesionsand caries with reduced false alarms.

In one embodiment, dental issue identifier 115 additionally includes anappearance comparator 172. Appearance comparator 172 determinesdifference in appearance (e.g., not based on changes in shape orphysical position) between the third representation or additionalrepresentation that is output by spatial comparator 168, imagedifference separator 178 and/or representation generator 179 and a firstrepresentation from the first image data 162. Alternatively, appearancecomparator 172 may compare a first representation of the dental archfrom the first image data 162 to a second representation of the dentalarch from the second image data 163 to determine appearance differences.As used herein, appearance includes color, hue, intensity, purity(whiteness), speculator or diffusion, percolation, transparency ortranslucency, and/or other visual indicators. Appearance may alsoinclude fluorescence level, which may be determined by shining a shortwavelength light into a tooth and receiving a longer wavelength lightthat is then emitted by the tooth. For example, a blue light may beemitted and blue to green light may be received, red light may beemitted and red to near infrared light may be received, or ultravioletlight may be emitted and ultraviolet to visible light may be received.Appearance differences are detected by comparing a generatedrepresentation to which rigid and non-rigid transformations have beenapplied to the first representation from the first image data 162 toidentify differences in appearances between regions of the dental archat a first time and the same regions of the dental arch at a secondtime.

Each point of the dental arch from one representation is compared to thesame point in the additional representation of the dental arch. Once thepoint and/or surface correspondence is complete, appearance comparator172 can subtract the color values (or other appearance values) of thereference surface from those of the test surface. The differences can beshown as a difference map generated by representation generator 179.

In some instances, scanners may introduce scanner inaccuracy to theappearance, such as color changes from scan to scan. Accordingly, in oneembodiment appearance comparator 172 performs a color calibration toeliminate non-clinical appearance differences that are attributable toscanner inaccuracy or differences in scanners. The color calibration maycorrect for changes of illumination with distance, changes ofillumination with angle and changes within the FOV of the scanner.Scanning differences are primarily caused by distance and angle of imagecapture. To overcome scanning differences, appearance comparator 172 mayuse a predefined or learned reference model to estimate parameters in asmall region. Alternatively, appearance comparator 172 may apply a modelthat takes into account stray light, self-illumination and percolationeffects of a generated image.

After either technique of eliminating color differences due to scannerinaccuracy is applied, some residual color and other appearancedifferences will remain. A few techniques may be used to remove suchresidual differences, either alone or in combination. One such techniqueis to apply a low pass filter on the three dimensional color mapgenerated based on the appearance comparison between the representationsof the dental arch. The low pass filter may separate out low frequencychanges (e.g., with a spatial frequency of 10 mm and less) that may bedue to scanner inaccuracies.

Another technique of eliminating color differences due to scannerinaccuracy is to determine a smooth function that when applied to onerepresentation will cause that representation to have the appearance ofthe other representation. To learn the smooth function, appearancecomparator 172 may take all matching points in an area, and perform amean square error (MSE) estimate or L1 estimate to a function to bringall points as near as possible to the other points. For example, thefollowing functions may be solved for:R ₁ =f(R ₂ ,G ₂ ,B ₂ ,x,y)G ₁ =f(R ₂ ,G ₂ ,B ₂ ,x,y)B ₁ =f(R ₂ ,G ₂ ,B ₂ ,x,y)Where x and y are the coordinates of a point on the dental arch, R₁, G₁and B₁ are the color values of a point (x,y) in the firstrepresentation, and R₂, G₂ and B₂ are the color values of the point inthe second representation. A polynomial function (e.g., a second degreeor third degree polynomial function) may also be used as the smoothingfunction. Once the smoothing function is generated, it may be applied toa color map of one representation of the dental arch to cause it to havecolors that more closely align to colors of the second representation.

Another technique of eliminating color differences due to scannerinaccuracy is to normalize the surface appearance of points in eachrepresentation to local environments for those points. For thistechnique, appearance comparator 172 identifies a tooth or pointneighboring region, and builds a normalized map. A region which isidentical to its neighbor will have a zero value in the normalized map.A normalized map may be generated for each representation, and the twonormalized maps may be compared rather than the raw color maps of thetwo representations.

Another technique of eliminating color differences due to scannerinaccuracy is to generate color models of each representation, and thengenerate additional maps that are based on derivatives of the colormaps. These maps of the derivatives can then be compared to identifychanges in appearance between the two representations. This may removeslow change colors that may be an error introduced by scanners.

Remaining appearance differences may be accentuated in the same manneras described above with reference to spatial differences. For example, acolor overlay may be generated, flags may be used, appearancedifferences may be extrapolated into the future, and so on. In anexample, a border or contour of a region that experiences a color changemay be added to a visual overlay to call attention to the appearancechange.

FIGS. 2-3B, 6, 8 and 10 below describe example applications ofdetermining dental issues of a patient based on comparison of image datataken at different times. The examples are described with reference toflow charts describing processes of determining dental issues. The flowcharts provide example processes that may be performed by system 100 ofFIG. 1A and/or by other computing devices. The methods depicted in FIGS.2-3B, 6, 8 and 10 may be performed by a processing logic that maycomprise hardware (e.g., circuitry, dedicated logic, programmable logic,microcode, etc.), software (e.g., instructions run on a processingdevice to perform hardware simulation), or a combination thereof.

FIG. 2 illustrates a flow diagram for a method 200 of identifying dentalissues, in accordance with an embodiment. At block 210 of method 200,processing logic receives first image data of a dental arch generated ata first time. The first image data may be a first three-dimensionalrepresentation of the dental arch that was generated based on anintraoral scan taken at the first time. At block 215, processing logicreceives second image data of the dental arch generated at a secondtime. The second image data may be a second three-dimensionalrepresentation of the dental arch that was generated based on anintraoral scan taken at the second time.

At block 218, processing logic compares the first image data to thesecond image data. This may include comparing the firstthree-dimensional representation from the first image data to the secondthree-dimensional representation from the second image data andregistering the first representation to the second representation asdiscussed above with reference to FIG. 1B. Given two surfaces (e.g., thefirst three-dimensional representation and the second three-dimensionalrepresentation of the dental arch), one of them being a test surface andthe other being a reference surface, processing logic determines thedifferences between the test surface and the reference surface. Any ofthe aforementioned techniques for comparing the first representation(e.g., reference surface) to the second representation (e.g., testsurface) described above may be applied. For example, techniques such assurface overlay and coloring of a test surface according to distancefrom points on the test surface to nearest points on the referencesurface may be used. Both approaches depend on a fine alignment of thetest surface to the reference surface that is typically achieved bysemi-automatic algorithms because for significantly mismatching surfacesdifferent alignment options are possible and it is difficult to fullyautomate the alignment process. One reason for this difficulty is thatstandard image registration methods do not handle situations where thereare several kinds of differences between surfaces of a significantlyvarious scale. Accordingly, standard image alignment and imageregistration methods are not generally applicable for comparison between3D representations of a dental arch generated by an intraoral scanner.However, in embodiments described herein the alignment and imageregistration process is fully automated and accommodates the multipledifferent scales of differences between representations of the dentalarch.

At block 220, processing logic determines spatial differences betweenthe first representation and the second representation that areattributable to scanner inaccuracy. At block 222, processing logicdetermines spatial differences attributable to clinical changes in thedental arch. Spatial differences attributable to scanner inaccuracy andspatial differences attributable to clinical changes may each beidentified based on a scale (e.g., spatial frequency) of the changes.Spatial differences having a large scale (and thus a low spatialfrequency) such as differences in arch length and jaw width may beidentified as global differences that are caused by scanner inaccuracy.Changes in arch width have a scale (and spatial frequency) of about 5-10cm and a magnitude of about 0.5-5.0 mm. Spatial differences havingsmaller scale (and thus a higher spatial frequency) such as differencesin tooth movement, tooth wear, gum recession, and gum swelling may beidentified as local differences that are caused by clinical changes tothe dental arch. Changes in teeth position have a scale (and spatialfrequency) of about 1-2 cm and a magnitude of about 0.3-5.0 mm. Changesin gingiva or gum recession and inflammation have a scale (and spatialfrequency) of about 1-5 mm and a magnitude of about 0.2-0.3 mm. Changesin tooth wear and changes in tooth shape due to restorations (e.g.,crowns, fillings, etc.) have a scale (and spatial frequency) of about0.2-1.0 mm and a magnitude of about 0.03-0.3 mm. With traditionalcomparison visualization methods, large-scale changes such as teethmovements or jaw expansion nearly completely hide smaller changes suchas tooth wear. However, the multilevel approach to surface matching andimage registration performed by processing logic enables the separationof differences between surfaces into several levels, and further enablesthe visualization of changes of different scales separately. Suchseparation can be achieved because changes such as tooth wear arestrongly localized and thereby can be separated from larger, smoother,lower frequency changes such as teeth movement or arch expansion. Anadditional benefit is that processing logic may perform imageregistration and determine surface differences in a completely automaticfashion that does not require manual intervention by a user in thesurface alignment process.

At block 223, processing logic may classify the spatial differencesattributable to clinical changes. For example, processing logic maydetermine the scale or spatial frequency for each of the determineddifferences, and classify those differences as one of changes in teethposition, changes in the gingiva (e.g., gum recession or guminflammation), changes in tooth shape (e.g., tooth wear andrestorations), and so on based on the scale or spatial frequency.Spatial differences may additionally or alternatively be classifiedusing one or more specific dental classification rules or algorithms.For example, a first classification rule may identify changes to thegingiva, a second classification rule may determine changes to toothshape, a third classification rule may determine tooth movements, and soon.

At block 224, processing logic generates a third representation of thedental arch that is a modified version of the second representation ofthe dental arch. Spatial differences attributable to scanner inaccuracymay be removed in the third representation. Accordingly, the remainingspatial differences are those that are attributable to clinical changein the dental arch. At block 230, processing logic updates the thirdrepresentation to include visual enhancements that accentuate thespatial differences attributable to clinical changes. This may includegenerating a visual overlay such as a color overlay and or extrapolatingthe spatial differences into the future. At block 235, processing logicoutputs the third representation of the dental arch. For example,processing logic may display the third representation of the dental archfor view by a dental practitioner or patient.

FIG. 3A illustrates a flow diagram for a method 300 of registering imagedata of a dental arch (e.g., performing image alignment) and identifyingand classifying dental issues, in accordance with an embodiment. Atblock 305 of method 300, processing logic selects a set of points on atest surface from the first image data of the dental arch. The testsurface may be a surface of a first three-dimensional representation ofthe dental arch generated based on a first intraoral scan. Suitablemethods for selecting the set of points include random sampling ofsurface vertices, binning of vertices to a voxel grid and averaging ineach voxel of the voxel grid, and feature detection (e.g., such as teethcusp detection). FIG. 4A is a diagram illustrating first image data forthe dental arch that shows the test surface 400 of the 3D representationof the dental arch. FIG. 4B is a diagram illustrating the test surface405 of the dental arch of FIG. 4A after points 408 on the test surfaceare selected, in accordance with an embodiment.

Returning to FIG. 3A, at block 310 processing logic associates eachpoint on the test surface with a small region on the test surfaceadjacent to that point. One possible suitable method that may be usedfor selecting these small regions (referred to as surface patches) is toselect parts of the surface that are less than a threshold distance awayfrom a selected point, and keeping a connected surface patch thatcontains the point itself. Another suitable technique is to performtooth segmentation and associate each point with a tooth crown thatincludes the selected point or a part of such tooth crown. FIG. 4C is adiagram illustrating the test surface 410 of the dental arch of FIG. 4Bshowing a particular region or surface patch 412 around a selected point413 on the test surface 410, in accordance with an embodiment.

Returning to FIG. 3A, at block 315 processing logic discards points thatcannot be associated with a region that satisfies one or more regioncriteria. For example, points that cannot be associated with asufficiently good region of a surface are discarded. A first regioncriterion may be a surface patch size threshold. Points that cannot beassociated with a surface patch having at least a minimum size may notsatisfy the surface patch size threshold a second region criterion andmay be discarded. A second region criterion may be a dental featurecriterion. Points that cannot be associated with a tooth crown, forexample, may fail to satisfy the dental feature criteria and may bediscarded.

At block 320, processing logic matches points on the test surface fromthe representation of the dental arch in the first image data to areference surface from the representation of the dental arch in thesecond image data. One matching technique that may be used includesrunning an iterative closest point (ICP) algorithm from several staringpositions. Another matching technique includes detecting a number oforientation-independent surface features on test surfaces of one imageand reference surfaces on the other image. For each feature on a testsurface, processing logic may find all similar features on the referencesurface and vote for particular transforms that would align features onthe test surface with the features on the reference surface. Thetransformation with the most votes may then be picked, and a result maybe refined using an ICP algorithm. FIG. 4D is a diagram illustratingmatching of the particular region or surface patch 412 of FIG. 4C to areference surface 417 from the second image data for the dental arch, inaccordance with an embodiment.

Returning to FIG. 3A, at block 325 processing logic validates thequality of detected matches with a suitable method and discards pointsthat did not match well. One suitable method for validation includescomputing a percentage of surface area on a tested part of the testsurface that is less than a threshold distance (e.g., in millimeters)away from the reference surface after alignment. A match may bevalidated if the size of the surface area that matched is larger than asize threshold. Another suitable method for validation includescomputing an average or median distance between vertices of a testedpart of the test surface and the reference surface after alignment. Ifthe average or median between vertices is less than a thresholddistance, then validation may be successful.

At block 330, processing logic computes a mean and/or median alignmentof the entire set of matches. Processing logic may detect and removeoutliers that suggest variants of alignment of the test surface andreference surface that are too different from the mean or medianalignment of the entire set of matches by using an appropriate method.For example, surface patches for which the alignment of the test surfaceto the reference surface has an alignment value that differs from themean or median by more than a threshold may be too different. Oneappropriate method that may be used is the random sample consensus(RANSAC) algorithm. If two surfaces are not comparable, then processinglogic will determine that the registration has failed based the numberof surviving points would be too small to reasonably cover the entiretest surface. This might happen, for example, if input data contained amistake (e.g., a user tried to match an intraoral scan of one person toan intraoral scan of another person). Processing logic may check forsuch a condition and report an error if this occurs.

A result of the surface matching may be a dense set of pairs of matchingpoints, with each pair corresponding to a region on the test surface anda matching region on the reference surface. Each such region is alsoassociated with a point, so each pair can also be viewed as a pair of apoint on the test surface and a matching point on reference surface. Anordered set of these points on a test surface is a point cloud on thetest surface, and a set of matching points on a reference surfaceordered in the same way is a matching point cloud on the referencesurface.

At block 335, processing logic computes an approximate alignmenttransformation to align a test point cloud for the test surface to areference point cloud for the reference surface. One example of asuitable algorithm for computing the alignment transformation is theleast-squares minimization of distance between the test and referencepoint clouds. The approximate alignment transformation may be a rigidalignment transformation. After approximate alignment via a rigidtransformation of the test surface, test and reference point cloudswon't coincide exactly. FIG. 4E illustrates a mapping 420 of points 425on the test surface 400 to corresponding points 430 on the referencesurface 417.

At block 340, processing logic computes a non-rigid transformation suchas a piecewise-smooth warp space transformation that smoothly deforms a3D space of the test surface such that 1) this deformation is as smoothas possible and b) the test point cloud after application of the warptransformation is much better aligned with a reference point cloud.Possible implementation options include, but are not limited to, radialbasis function interpolation, thin-plate splines (TPS) and estimatingteeth movements and propagating them to a nearby space. FIG. 4F is adiagram illustrating a warp space transformation between first imagedata (test surface) 440 of a dental arch and second image data(reference surface) 450 of the dental arch, in accordance with anembodiment. A magnitude of the warp space transformation may beidentified as a global component of spatial differences that areattributable to scanner inaccuracy. Accordingly, the warp spacetransformation may identify the spatial differences caused by scannerinaccuracy.

Once corresponding point sets are determined between surface patches ofthe two images, determination of the transformation between the two setsof corresponding points in two coordinate frames can be solved.Essentially, an image registration algorithm may compute atransformation between two images that will minimize the distancesbetween points on one surface, and the closest points to them found inthe interpolated region on the other image surface can be used as areference. The transformation may include rotations and/or translationalmovement in up to six degrees of freedom. Additionally, thetransformation may include deformation of one or both of the images(e.g., warp space transformations and/or other non-rigidtransformations). A result of the image registration may be one or moretransformation matrix that indicates the rotations, translations and/ordeformations that will cause the one image to correspond to the otherimage.

At block 345, processing logic applies an alignment transformation and anon-rigid (e.g., warp space) transformation to the test surface. Atblock 350, processing logic computes residual differences between thealigned and warped test surface and the reference surface. This residualmay be reported as a “local” component of the surface difference that isattributable to clinical changes in the dental arch. A possibleimplementation option includes computing a distance to a nearest pointof the reference surface for each point of an aligned and warped testsurface and then coloring the test surface accordingly (e.g., coloringbased on a magnitude of the distance, where a first color may be usedfor a first distance magnitude, a second color may be used for a seconddistance magnitude, and so on). Another possible implementation optionincludes performing an additional comparison as set forth above to splitthe remaining differences into additional levels (e.g., into toothmovement, changes in gingival shape, changes to the tooth surface, andso on). For example, the operations of blocks 305-345 may be repeated.In a first repetition of the operations of blocks 305-345, the testsurface that may be used is the aligned and warped test surface (inwhich the spatial differences caused by scanner inaccuracy have beenremoved). As a result, spatial differences caused by tooth movement maybe the spatial differences with the largest scale or spatial frequencythat remain. An additional warp space transformation may show thespatial differences caused by tooth movement. In a second repetition ofthe operations of blocks 305-345, the spatial differences with the nextlargest scale or spatial frequency may be identified, and so on.

At block 355, processing logic generates a representation of the dentalarch that shows some or all of the differences between the test surfaceand the reference surface. In one embodiment, alignment transformation(e.g., transformation in position and/or orientation) and warp spacetransformation (e.g., deformation) computed from a point cloud may beapplied to the entire test surface. A magnitude of the warp spacetransformation may be reported as a “global” component of a surfacedifference and visualized with a suitable method. The differences causedby scanner inaccuracy as represented in the warp space transformationmay be visualized using a suitable method. Suitable methods include aper-point coloring of the test surface according to a magnitude of thewarp movement at this point and coloring of an entire tooth crownaccording to a magnitude of its movement. Such global component of thesurface difference may be the surface difference that is attributable toscanner inaccuracy. Similar visualization techniques can be used foreach of the different levels. Multiple levels of spatial differences maybe shown together (e.g., each with different coloring or other differentvisualization. Alternatively, a user may turn on or off visualization ofone or more of the levels. The visualization may highlight or accentuateregions of significant changes. Additionally, the differences may befiltered to hide insignificant changes and simplify analysis. In oneembodiment, a dental practitioner may specify thresholds of importancefor each type of clinical change. For example, a dental practitioner mayspecify a single threshold or a lower and upper threshold for changes inthe gingival line, for tooth movement, for tooth wear, and so on. Afilter may be used to filter out those changes that have a smallermagnitude than a lower threshold and/or to emphasize those changes thathave a higher magnitude than the upper threshold. Default thresholds mayalso be used. In one example, spatial differences having a magnitudethat is less than a lower threshold may be shown with a first color,spatial differences having a magnitude between the lower and upperthresholds may be shown with a second color, and spatial differenceshaving a magnitude that is greater than the upper threshold may be shownwith a third color.

FIG. 3B illustrates a flow diagram for a method 356 of comparing imagedata of a dental arch and identifying and visualizing dental issues, inaccordance with an embodiment. The method 356 begins with receiving areference surface (first image data of a dental arch) 357 and receivinga test surface 358 (second image data of the dental arch). Method 356 isdescribed with reference to making modifications to the test surface 358for comparison and matching with the reference surface 357. However, itshould be understood that modifications may instead be made to thereference surface 357 for comparison and matching with the test surface358.

At block 360, a rigid body alignment is determined for alignment of thetest surface 358 to the reference surface 357. At block 362, surfacepositions from the rigid body alignment are tested. At block 318, thetest surface is modified using the rigid body transformation. At block364, the test surface is therefore aligned with the reference surfaceusing the rigid body alignment. A visualization of the alignment may beshown at block 370.

At block 366, a jaw scale change detector is applied to identify largescale differences of changes in the jaw scale (e.g., jaw width) or archlength between the modified test surface and the reference surface. Thejaw scale detector may be a first low pass filter. At block 368, asmooth vector field is determined for a non-rigid alignment between therigidly aligned test surface 358 and reference surface 357. At block328, the test surface is further modified using the smooth vector field.

At block 372 the test surface 358 is therefore matched to the arch widthof the reference surface 357. The overlay of the jaw visualization maythen be output at block 370. If an overlay was earlier output, theoverlay may be updated.

At block 378, the test surface 358 as modified at block 372 is comparedto the reference surface 357 and processed by a teeth movement detector.The teeth movement detector may be another low pass filter having ahigher spatial frequency threshold than the first low pass filter. Atblock 380, teeth movement information is determined based on an outputof the teeth movement detector. The teeth movement information, modifiedtest surface and/or a treatment plan 374 may then be used to show avisualization of tooth motion. The treatment plan 374 may indicateplanned tooth motion, which may be compared to the actual detected toothmotion. Differences between the planned tooth motion and the actualtooth motion may then be shown in the tooth motion visualization atblock 376.

At block 338, the teeth movements are subtracted from the modified testsurface to further modify the test surface. At block 382, a test surfacethat has been compensated for teeth movements is then generated. Atblock 384, a gum recession visualization may be output.

At block 386 the test surface as modified at block 382 may be comparedto the reference surface 357 using nearest point matching. At block 388,point-to-point surface matches may be performed between the modifiedtest surface and the reference surface. At block 390, a low pass filteris applied to the surface matches to determine changes attributable totooth wear. A tooth wear visualization may then be output at block 391.

At block 348, the test surface 358 is further modified using a result ofthe point-to-point surface matches. At block 392, a color differenceanalysis is performed between the modified test surface and thereference surface. At block 394, a white balance compensation isdetermined for the test surface. At block 349, the test surface isfurther modified by applying the white balance compensation. At block395, a white balance-compensated color difference is determined betweenthe modified test surface and the reference surface. At block 396, a lowpass spatial filter may then be applied, and a color differencevisualization may be output at block 397.

FIGS. 5A-5D illustrate visual maps showing correlation between firstimage data of a dental arch and second image data of the dental arch, inaccordance with an embodiment. In these figures spatial differences havebeen divided into global and local scales (where each scale represents adifferent class of spatial difference). FIG. 5A illustrates an overlaybetween a test and reference surface. FIG. 5B illustrates a traditionalpoint-to-point difference between the test surface and the referencesurface with differences appearing on a scale of 20-300 microns. FIG. 5Cshows global differences between the test and reference surfaces thatare attributable to scanner inaccuracy with differences appearing on ascale of 100-300 microns. FIG. 5D shows local differences between thetest and reference surfaces that are attributable to clinical changes ona scale of 20-100 microns. The global differences of FIG. 5C show thatthe molars have the greatest difference between surfaces. However, thelocal differences of FIG. 5D show that the fourth tooth in from theright has the most change (e.g., due to grinding tooth moving relativeto a neighboring tooth). A dental practitioner looking at any of thevisual maps of FIGS. 5A-5C may not easily identify that tooth wear hasoccurred on the fourth tooth from the right. However, the visual map ofFIG. 5D clearly shows to the dental practitioner the clinical changes tothis tooth.

FIG. 6 illustrates a flow diagram for a method 600 of determiningappearance differences between first image data of a dental arch andsecond image data of the dental arch, in accordance with an embodiment.At block 618 of method 600, processing logic compares a correctedrepresentation of a dental arch to an additional representation of thedental arch. The corrected representation may be a representation thatwas generated after comparison of a first representation from firstimage data to a second representation from second image data, where thecorrected representation is a modified version of the secondrepresentation in which differences between the first and secondrepresentations that are attributable to scanner inaccuracy have beenremoved. For example, the corrected representation may be arepresentation that is generated after performing the operations ofmethod 200 or method 300. The additional representation may be arepresentation from first image data of the dental arch, which may havebeen generated based on a first intraoral scan. At block 620, processinglogic determines appearance differences between the correctedrepresentation in the additional representation that are attributable toscanner inaccuracy. At block 625, processing logic determines appearancedifferences attributable to clinical changes in the dental arch. Atblock 630, processing logic may classify the appearance differencesattributable to clinical changes. Classification may be performed byapplying one or more classification rules, where each classificationrule may detect a different type of clinical change (e.g., appearancedifferences in the gums vs. appearance differences in the teeth).

At block 635, processing logic updates the corrected representation ofthe dental arch. The appearance differences that are attributable toscanner inaccuracy are removed in the corrected representation. At block640, processing logic updates the corrected representation to includevisual enhancements that accentuate the differences that areattributable to clinical changes. At block 645, processing logic outputsthe corrected representation of the dental arch.

FIG. 7A illustrates a tooth before a change in appearance of the tooth700 and a tooth after the change in appearance of the tooth 750. Afterthe change in appearance has occurred, the tooth includes a white area755 that was not present before the appearance change. Such anappearance change may be detected by method 600.

FIG. 7B illustrates a first representation 760 of a set of teeth and asecond representation 770 of the set of teeth. The first representationwas generated from image data taken at a first time and the secondrepresentation 770 was generated from image data taken at a second time.As shown, an appearance to one of the teeth 765 has changed between thefirst representation 760 and the second representation 770.Specifically, the translucency of the tooth 765 has increased at aregion 775 of the tooth 765. Such an appearance change may be detectedby method 600.

FIG. 8 illustrates a flow diagram for a method 800 of generating arepresentation of a dental arch that accentuates changes that haveoccurred in the dental arch, in accordance with an embodiment. At block825 of method 800, processing logic determines spatial and/or appearancedifferences between first image data (e.g., a first representation) andsecond image data (e.g., a second representation) of a dental arch thatare attributable to clinical changes in the dental arch. Aftercompletion of method 200 and/or method 300, a point to point matchingmay have been determined (after applying smooth deformation to a testsurface).

At block 830, processing logic determines an elapsed time between whenthe first image data and the second image data were generated. At block835, processing logic determines rates of change associated with thespatial and/or appearance differences. At block 838, processing logicextrapolates the spatial and/or appearance differences into the futurebased on the determined rates of change. Processing logic mayadditionally or alternatively interpolate the differences between whenthe first and second image data was taken.

At block 840, processing logic updates a representation of the dentalarch to include extrapolated or interpolated spatial and/or appearancechanges. At block 845, processing logic outputs the representation ofthe dental arch showing the extrapolated or interpolated spatial and/orappearance changes.

FIG. 9 illustrates extrapolated positions of a set of points on a dentalarch 900. As shown, each of points p1, p2, p3 and p4 have a firstposition at time t0 and a second position at a time t1. Extrapolationmay be performed to determine a third position of the points p1, p2, p3and p4 at an extrapolated time in the future.

A set of surfaces from past to future can be created to make an animatedvideo of one or more clinical changes associated with the detectedappearance and/or spatial differences in embodiments. The future portionof the video may also serve as an exaggeration of the clinical changes.Different frames of the video that show the differences between the twotimes may be generated by the interpolation, and additional frames ofthe video that show the differences after the second time may begenerated by the extrapolation. The movement between frames may becontrolled by a slider, showing the evolution of the clinical changesover time. If more than two measurements exist, interpolation betweenthe multiple images may be used or a higher order extrapolation and/orinterpolation may be used to increase an accuracy of the clinicalchanges over time. Techniques such as a parabolic function, splines,etc. may be used for this purpose.

Interpolating and extrapolating the trajectory of the appearance andspatial differences on a point by point basis can be prone to error andnoise. Such error and noise may be reduced in embodiments by computingsurface patches and computing trajectories of the surface patches.Processing logic may take small surface patches (e.g., 500×500 micronsurface patches) containing many points and compute sharedtransformations between subsets of the surface patches. Thetransformation can be a non-rigid transformation, a rigidtransformation, an affine transformation, and/or other transformation.The transformations can be restricted to be continuous to one another,or continuous and smooth (e.g., by comparing derivatives of thetransformations between adjacent surface patches).

There are a number of different types of surface changes or movementthat may be shown. Processing logic may use a different technique tovisualize each type of surface change or movement. Tooth movement may beconsidered as a rigid body movement. To show tooth movement, a smoothnon-rigid transformation (smooth deformation) may have been computed asset forth in methods 200 and 300. Additionally, tooth segmentation maybe performed to identify individual teeth. Each tooth motion may then beestimated as an independent rigid body. The rigid body transformationfor each tooth that has moved may be decomposed into translations (orposition shifts) and rotations. To generate a trajectory of a tooth,processing logic may determine an interpolated transform between aninitial reference position of the tooth and the final full motion of thetooth by rotating a partial angle and performing a partial translation.For an enhanced change, processing logic continues to apply therotations and translations and project them to future times (e.g., usingexponential mapping, matrix exponentiation, and so on).

Tooth grinding happens at specific portions of teeth (e.g., upper partof a tooth where contact occurs with teeth on an opposing jaw). Tovisualize tooth wear, processing logic may use a point to pointcomparison technique, but limit the change to the tooth shape to surfacesmoothness operations. Processing logic may interpolate to findintermediate tooth shapes and extrapolate to find future tooth shapesbased on increasing the smoothness of the tooth.

Other techniques for interpolation and extrapolation may be used forvisualizing other types of clinical changes such as tartar accumulation,sum swelling, gum recession, color change, and so on.

Changes that occur over time can be presented in various interfaces,including a side-to-side comparison, color mapping, an animation overtime, a timeline slider, and so on. The timeline can include possible,predictive future changes and can have multiple paths based on treatmentoptions selected by the dental practitioner.

After clinical changes are detected, processing logic may predict ifclinical issues will occur in the near future as a result of the changescontinuing. The extrapolation of the changes into the future may beapplied to one or more thresholds to determine if clinical changes willbe problematic for a patient. For example, a clinical change may not beproblematic at present. However, extrapolation into the future may showthat the clinical change will reach a critical point if it continuesinto the future. The critical point may be identified based on the oneor more thresholds. For example, if tooth erosion will exceed a tootherosion threshold at a future time, then that tooth erosion may beidentified as problematic. This may assist a dental practitioner to takeproactive measures like prescribing aligners, prescribing night guards,and so on.

In some embodiments, processing logic may track treatment milestones totrack improvements or worsening conditions as a result of thosemilestones. Examples of milestones include crown delivery, orthodontictreatment, night guard delivery, tooth implant, and so on.

In some embodiments, the severity of detected spatial and appearancedifferences caused by clinical changes may be determined based oncomparison to one or more thresholds. Such thresholds may be defaultvalues or selected by a dental practitioner. Such clinical changes maythen be visualized based on the determined severity. For example, moresevere clinical changes may be shown in a first color and less severeclinical changes may be shown in a second color. For example, hot colorssuch as red may show severe changes (e.g., tooth wear greater than 0.2mm) and cold colors such as blue may show less severe changes (e.g.,tooth wear of 0.05 mm or less). Other display options include addingbars that extend from a point of interest (e.g., a region of the dentalarch associated with a detected clinical change), where the width of thebar is based on a severity of the clinical change. A user may be able toselect the type of clinical change that is of interest, and only theselected type of clinical change may be shown for that selection.

FIG. 10 illustrates a flow diagram for a method 1000 of generating avisual overlay for a virtual model of a dental arch, in accordance withan embodiment. The visual overlay may show compliance with anorthodontic treatment plan and deviation from the orthodontic treatmentplan, in accordance with an embodiment. The visual overlay mayadditionally or alternatively show clinical changes to regions of apatient's dental arch. Method 1000 may be performed at the end of method200. Method 1000 may also be performed in conjunction with method 300.At block 1010 of method 1000, processing logic generates a virtual modelof the dental arch. The virtual model of the dental arch may be ageneric model or may be a model tailored to a patient's specific dentalarch (e.g., generated from an intraoral scan of the patient's dentalarch). At block 1015, processing logic generates an overlay for thevirtual model that identifies clinical signs that have been identified.The overlay may be a graphical overlay such as a color overlay thatcolor codes teeth on the dental arch according to clinical signsassociated with those teeth. At block 1020, processing logic outputs thevirtual model and the overlay. Accordingly, the dental practitioner mayeasily determine what clinical changes have a been identified and where,and may at a glance determine how an orthodontic treatment plan isprogressing.

FIG. 15 illustrates a diagrammatic representation of a machine in theexample form of a computing device 1500 within which a set ofinstructions, for causing the machine to perform any one or more of themethodologies discussed herein, may be executed. In alternativeembodiments, the machine may be connected (e.g., networked) to othermachines in a Local Area Network (LAN), an intranet, an extranet, or theInternet. The machine may operate in the capacity of a server or aclient machine in a client-server network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet computer, a set-topbox (STB), a Personal Digital Assistant (PDA), a cellular telephone, aweb appliance, a server, a network router, switch or bridge, or anymachine capable of executing a set of instructions (sequential orotherwise) that specify actions to be taken by that machine. Further,while only a single machine is illustrated, the term “machine” shallalso be taken to include any collection of machines (e.g., computers)that individually or jointly execute a set (or multiple sets) ofinstructions to perform any one or more of the methodologies discussedherein. In one embodiment, the computer device 1500 corresponds tocomputing devices 155 of FIG. 1A.

The example computing device 1500 includes a processing device 1502, amain memory 1504 (e.g., read-only memory (ROM), flash memory, dynamicrandom access memory (DRAM) such as synchronous DRAM (SDRAM), etc.), astatic memory 1506 (e.g., flash memory, static random access memory(SRAM), etc.), and a secondary memory (e.g., a data storage device1528), which communicate with each other via a bus 1508.

Processing device 1502 represents one or more general-purpose processorssuch as a microprocessor, central processing unit, or the like. Moreparticularly, the processing device 1502 may be a complex instructionset computing (CISC) microprocessor, reduced instruction set computing(RISC) microprocessor, very long instruction word (VLIW) microprocessor,processor implementing other instruction sets, or processorsimplementing a combination of instruction sets. Processing device 1502may also be one or more special-purpose processing devices such as anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), a digital signal processor (DSP), network processor,or the like. Processing device 1502 is configured to execute theprocessing logic (instructions 1526) for performing operations and stepsdiscussed herein.

The computing device 1500 may further include a network interface device1522 for communicating with a network 1564. The computing device 1500also may include a video display unit 1510 (e.g., a liquid crystaldisplay (LCD) or a cathode ray tube (CRT)), an alphanumeric input device1512 (e.g., a keyboard), a cursor control device 1514 (e.g., a mouse),and a signal generation device 1520 (e.g., a speaker).

The data storage device 1528 may include a machine-readable storagemedium (or more specifically a non-transitory computer-readable storagemedium) 1524 on which is stored one or more sets of instructions 1526embodying any one or more of the methodologies or functions describedherein, such as instructions for an AR processing module 1550. Anon-transitory storage medium refers to a storage medium other than acarrier wave. The instructions 1526 may also reside, completely or atleast partially, within the main memory 1504 and/or within theprocessing device 1502 during execution thereof by the computer device1500, the main memory 1504 and the processing device 1502 alsoconstituting computer-readable storage media.

The computer-readable storage medium 1524 may also be used to store adental issue identifier 1550, which may correspond to the similarlynamed component of FIGS. 1A-1B. The computer readable storage medium1524 may also store a software library containing methods for an dentalissue identifier 1550. While the computer-readable storage medium 1524is shown in an example embodiment to be a single medium, the term“computer-readable storage medium” should be taken to include a singlemedium or multiple media (e.g., a centralized or distributed database,and/or associated caches and servers) that store the one or more sets ofinstructions. The term “computer-readable storage medium” shall also betaken to include any medium other than a carrier wave that is capable ofstoring or encoding a set of instructions for execution by the machineand that cause the machine to perform any one or more of themethodologies of the present invention. The term “computer-readablestorage medium” shall accordingly be taken to include, but not belimited to, solid-state memories, and optical and magnetic media.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent upon reading and understanding the above description. Althoughembodiments of the present invention have been described with referenceto specific example embodiments, it will be recognized that theinvention is not limited to the embodiments described, but can bepracticed with modification and alteration within the spirit and scopeof the appended claims. Accordingly, the specification and drawings areto be regarded in an illustrative sense rather than a restrictive sense.The scope of the invention should, therefore, be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The invention claimed is:
 1. A method comprising: making a comparisonbetween first image data of a dental arch and second image data of thedental arch; determining, by a processing device, a plurality of spatialdifferences between a first representation of the dental arch in thefirst image data and a second representation of the dental arch in thesecond image data; determining, by the processing device, that a firstspatial difference of the plurality of spatial differences isattributable to scanner inaccuracy; determining, by the processingdevice, that a second spatial difference of the plurality of spatialdifferences is attributable to a clinical change to the dental arch; andgenerating, by the processing device, a third representation of thedental arch that is a modified version of the second representation,wherein the first spatial difference is removed in the thirdrepresentation, and wherein the third representation comprises a visualenhancement that accentuates the second spatial difference.
 2. Themethod of claim 1, further comprising: determining a classification forthe second spatial difference that is attributable to the clinicalchange, wherein the visual enhancement is based at least in part on theclassification.
 3. The method of claim 2, wherein the classification forthe second spatial difference is one of a tooth movement classification,a tooth wear classification, a gum recession classification, or a gumswelling classification.
 4. The method of claim 2, wherein determiningthe classification comprises: applying a plurality of detection rules tothe second spatial difference, wherein each of the plurality ofdetection rules identifies a specific type of clinical change.
 5. Themethod of claim 1, wherein: the first image data was generated based ona first intraoral scan of the dental arch performed at a first time by afirst intraoral scanner and the second image data was generated based ona second intraoral scan of the dental arch performed at a second time bythe first intraoral scanner or a second intraoral scanner; and an errorcomprises the scanner inaccuracy of at least one of the first intraoralscanner or the second intraoral scanner.
 6. The method of claim 1,wherein determining that the first spatial difference is attributable toscanner inaccuracy and that the second spatial difference isattributable to the clinical change to the dental arch comprises:applying a low pass filter that identifies low frequency spatialdifferences, wherein the first spatial difference is a low frequencyspatial difference having a first value that is below a frequencythreshold of the low pass filter and the second spatial difference is ahigher frequency spatial difference having a second value that is abovethe frequency threshold.
 7. The method of claim 1, wherein the firstspatial difference is a difference in arch length between the firstrepresentation of the dental arch and the second representation of thedental arch.
 8. The method of claim 1, wherein the visual enhancementcomprises a color overlay for the third representation of the dentalarch, wherein the color overlay comprises a first color for a region ofthe dental arch associated with the second spatial difference and asecond color for one or more additional regions of the dental arch forwhich no clinical differences were identified between the firstrepresentation and the second representation of the dental arch.
 9. Themethod of claim 1, further comprising: determining a magnitude of thesecond spatial difference; and comparing the magnitude of the secondspatial difference to one or more difference thresholds to determine aseverity of the clinical change, wherein the visual enhancement is basedat least in part on the severity of the clinical change.
 10. The methodof claim 9, further comprising: receiving an input defining the one ormore difference thresholds, wherein the one or more differencethresholds comprise an upper threshold and a lower threshold.
 11. Themethod of claim 1, further comprising: making an additional comparisonbetween the first representation and the third representation;determining a plurality of appearance differences between the firstrepresentation and the third representation, wherein an appearancedifference comprises a difference in at least one of color,translucency, transparency, hue or intensity between a region in thefirst representation and a corresponding region in the secondrepresentation; and adding an additional visual enhancement thataccentuates one or more of the plurality of appearance differences. 12.The method of claim 11, further comprising: determining that a firstappearance difference of the plurality of appearance differences isattributable to scanner inaccuracy of a scanner; determining that asecond appearance difference of the plurality of appearance differencesis attributable to a clinical change to the dental arch; and updating anappearance of the third representation to remove the first appearancedifference, wherein the additional visual enhancement accentuates thesecond appearance difference.
 13. The method of claim 1, furthercomprising: generating the visual enhancement that accentuates thesecond spatial difference by extrapolating the second spatial differenceto a future time, wherein the clinical change is more extensive afterextrapolation.
 14. The method of claim 1, wherein the firstrepresentation is a first virtual three-dimensional (3D) model of thedental arch, the second representation is a second virtual 3D model ofthe dental arch, and the third representation is a third virtual 3Dmodel of the dental arch.
 15. The method of claim 1, wherein the secondspatial difference comprises an actual tooth movement for a tooth, themethod further comprising: making a comparison of the actual toothmovement for the tooth to a planned tooth movement for the tooth;determining, based on the comparison of the actual tooth movement forthe tooth to the planned tooth movement for the tooth, whether theactual tooth movement deviates from the planned tooth movement by morethan a threshold amount; and generating a visual indicator thatindicates whether the actual tooth movement deviates from the plannedtooth movement by more than the threshold amount.
 16. A systemcomprising: a memory; and a processing device operatively coupled to thememory, the processing device to: make a comparison between first imagedata of a dental arch and second image data of the dental arch;determine a plurality of spatial differences between a firstrepresentation of the dental arch in the first image data and a secondrepresentation of the dental arch in the second image data; determinethat a first spatial difference of the plurality of spatial differencesis attributable to scanner inaccuracy; determine that a second spatialdifference of the plurality of spatial differences is attributable to aclinical change to the dental arch; and generate a third representationof the dental arch that is a modified version of the secondrepresentation, wherein the first spatial difference is removed in thethird representation, and wherein the third representation comprises avisual enhancement that accentuates the second spatial difference. 17.The system of claim 16, wherein the processing device is further to:determine a classification for the second spatial difference that isattributable to the clinical change, wherein the visual enhancement isbased at least in part on the classification, wherein the classificationis one of a tooth movement classification, a tooth wear classification,a gum recession classification, or a gum swelling classification. 18.The system of claim 17, wherein the processing device is further to:apply a plurality of detection rules to the second spatial difference todetermine the classification for the second spatial difference, whereineach of the plurality of detection rules identifies a specific type ofclinical change.
 19. The system of claim 16, wherein to determine thatthe first spatial difference is attributable to scanner inaccuracy andthat the second spatial difference is attributable to the clinicalchange to the dental arch the processing device is to: apply a low passfilter that identifies low frequency spatial differences, wherein thefirst spatial difference is a low frequency spatial difference having afirst value that is below a frequency threshold of the low pass filterand the second spatial difference is a higher frequency spatialdifference having a second value that is above the frequency threshold.20. The system of claim 16, wherein the visual enhancement comprises acolor overlay for the third representation of the dental arch, whereinthe color overlay comprises a first color for a region of the dentalarch associated with the second spatial difference and a second colorfor one or additional regions of the dental arch for which no clinicaldifferences were identified between the first representation and thesecond representation of the dental arch.
 21. The system of claim 16,wherein the processing device is further to: determine a magnitude ofthe second spatial difference; and compare the magnitude of the secondspatial difference to one or more difference thresholds to determine aseverity of the clinical change, wherein the visual enhancement is basedat least in part on the severity of the clinical change.
 22. The systemof claim 16, wherein the processing device is further to: make anadditional comparison between the first representation and the thirdrepresentation; determine a plurality of appearance differences betweenthe first representation and the third representation, wherein anappearance difference comprises a difference in at least one of color,translucency, transparency, hue or intensity between a region in thefirst representation and a corresponding region in the secondrepresentation; and add an additional visual enhancement thataccentuates one or more of the plurality of appearance differences. 23.The system of claim 22, wherein the processing device is further to:determine that a first appearance difference of the plurality ofappearance differences is attributable to scanner inaccuracy; determinethat a second appearance difference of the plurality of appearancedifferences is attributable to a clinical change to the dental arch; andupdate an appearance of the third representation to remove the firstappearance difference, wherein the additional visual enhancementaccentuates the second appearance difference.
 24. The system of claim16, wherein the second spatial difference comprises an actual toothmovement for a tooth, and wherein the processing device is further to:make a comparison of the actual tooth movement for the tooth to aplanned tooth movement for the tooth; determine, based on the comparisonof the actual tooth movement for the tooth to the planned tooth movementfor the tooth, whether the actual tooth movement deviates from theplanned tooth movement by more than a threshold amount; and generate avisual indicator that indicates whether the actual tooth movementdeviates from the planned tooth movement by more than the thresholdamount.
 25. The system of claim 16, wherein: the first image data wasgenerated based on a first intraoral scan of the dental arch performedat a first time by a first intraoral scanner and the second image datawas generated based on a second intraoral scan of the dental archperformed at a second time by the first intraoral scanner or a secondintraoral scanner; and an error comprises the scanner inaccuracy of atleast one of the first intraoral scanner or the second intraoralscanner.
 26. The system of claim 16, wherein the processing device isfurther to: generate the visual enhancement that accentuates the secondspatial difference by extrapolating the second spatial difference to afuture time, wherein the clinical change is more extensive afterextrapolation.
 27. A non-transitory storage medium comprisinginstructions that, when executed by a processing device, cause theprocessing device to perform operations comprising: making a comparisonbetween first image data of a dental arch and second image data of thedental arch, wherein the first image data was generated based on a firstintraoral scan of the dental arch performed at a first time by a firstintraoral scanner and the second image data was generated based on asecond intraoral scan of the dental arch performed at a second time bythe first intraoral scanner or a second intraoral scanner; determining,by a processing device, a plurality of spatial differences between afirst representation of the dental arch in the first image data and asecond representation of the dental arch in the second image data;determining, by the processing device, that a first spatial differenceof the plurality of spatial differences is attributable to scannerinaccuracy of at least one of the first intraoral scanner or the secondintraoral scanner; determining, by the processing device, that a secondspatial difference of the plurality of spatial differences isattributable to a clinical change to the dental arch; and generating, bythe processing device, a third representation of the dental arch that isa modified version of the second representation, wherein the firstspatial difference is removed in the third representation, and whereinthe third representation comprises a visual enhancement that accentuatesthe second spatial difference.